1. Field of the Invention
The present invention relates to a catalytic body preferably used for purification of components to be purified such as carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) contained in exhaust gas emitted from automobile engines, construction machine engines, industrial stationary engines, combustion devices, or the like, and its manufacturing method.
2. Description of the Related Art
Currently, a catalytic body utilizing a honeycomb structure as a catalyst carrier is used in order to purify exhaust gas emitted from various types of engines or the like. The catalytic body has a structure in which a catalyst coating layer 5 is carried on the surface of partition walls 4 defining cells 3 of a honeycomb structure, as shown in FIG. 13. The catalyst coating layer 5 mainly comprises an oxide such as alumina, ceria, or zirconia and a noble metal such as platinum, rhodium, or palladium is carried thereon as a catalytic active component. As shown in FIGS. 13 and 14, to purify exhaust gas by using the catalytic body 60 (honeycomb structure 11), exhaust gas flows into the cells 3 from the side of one end surface 2a so as to come into contact with the catalyst coating layer 5 on the inner surface of the partition walls 4, and then flows out of the cells from the side of the other end surface 2b to the outside (for example, refer to Patent document 1).
In case where exhaust gas is purified using such a honeycomb catalytic body, it is necessary to promote, as much as possible, the transmission of components to be purified contained in the exhaust gas from the exhaust gas toward the catalyst coating layer on the surface of the partition walls in order to enhance the purifying efficiency. To enhance the purifying efficiency, it is necessary to reduce the hydraulic diameter of cells and increase the surface area of partition walls. In particular, a method is employed for increasing the number of cells per unit area (cell density).
However, enhancing the cell density and reducing the hydraulic diameter of cells do not actually deliver full catalyst performance, and in order to further enhance the catalyst performance, there has recently been practiced a research for applying a honeycomb structure with plugged cells, such as one used in a diesel particulate filter (DPF), where the ends of cells are alternately plugged at one end surface and the other end surface of the honeycomb structure (so that each end surface of the honeycomb structure will appear in a zigzag pattern), to the catalyst carrier of the catalytic body.
That is, in a honeycomb structure where cells are plugged as described above, since exhaust gas flowing into the cells from the side of one end surface passes through porous partition walls and flows out of the cells from the side of the other end surface, if a catalyst coating layer including a catalytic active component such as a noble metal is coated on the inner surfaces of the fine pores in the partition walls through which exhaust gas passes, it is possible that the exhaust gas contacts the catalyst coating layer inside the fine pores of the partition walls to thus perform the purification of the exhaust gas. In a catalytic body having such a structure, the exhaust gas flows in the fine pores in the partition walls whose hydraulic diameter is smaller than that of a cell so that it is possible to reduce the distance between the exhaust gas and the catalyst coating layer and increase the contact area between the exhaust gas and the catalyst coating layer, with the result that drastic improvement of the catalyst performance is expected.
In the actual preparation of such a catalytic body, however, it is extremely difficult to uniformly coat a catalyst coating layer on the inner surfaces of the fine pores in the partition walls, and problems arise in that if the uniformity of a catalyst coating layer is not ensured, the catalyst performance is not fully achieved. Also, if an attempt is made to uniformly and thinly coat a catalyst coating layer on the inner surfaces of the fine pores in the partition walls, the volume of the catalyst coating layer itself, that is, the absolute quantity of an oxide constituting the catalyst coating layer is limited. Or, the distance between noble metal particles carried in or on the catalyst coating layer is insufficient so that the noble metal particles start to coagulate while the catalytic body is in use, which reduces the total surface area of the noble metal particles thus degrading the catalytic activity. On the other hand, if an attempt is made to sufficiently maintain an absolute quantity of the catalyst coating layer, the fine pores in the partition walls are excessively buried with the catalyst coating layer, and thus the passage resistance of exhaust gas grows excessively.
[Patent document 1] JP-A-2003-33664